Songfacts®:

• Lewis penned the title track of her third studio album with Ryan Tedder, who was also responsible for her debut single, "Bleeding Love." She recalled writing the song with the OneRepublic frontman during an interview with Digital Spy: "'Glassheart' we wrote, I remember, when I was a little bit down," she said. "I went to Denver to hang out with Ryan - to see him and his family - and see if we could come up with something nice. I remember just not wanting to sing a ballad or anything that would make me more sad. I was like, 'Can you give me a song where I can just dance and jump around?', and he was like, 'What about this?' - he played this beat and it was really hypnotic. So I was like, 'Yes! We have to do this'."
• Lewis told The Sun that this is "very different," from the material she recorded before. "It's kind of dubstep-inspired," she added. "I think the darker, dubstep thing works with my voice and I wanted to go there."
• Regarding the album title, Lewis explained to The Sun: "I was talking with Ryan Tedder about relationships and break-ups and he was like, 'You need to be careful with your heart, it's very fragile like a glass heart. 'I was like, 'Oh my God, that's what I should name my album, 'and then we wrote a song."